1. Field of the Invention
This invention relates generally to an electronic indenter system that measures the hardness and depth of a hardened portion of a part and, more particularly, to an electronic intelligent indenter system that measures the hardness and depth (case depth) of a hardened portion of a part in a non-destructive manner, where the system includes an electronic indenter tool, a laser for producing ultrasonic waves in the part, and an integral detector for detecting ultrasonic waves reflected from the transition boundary between the hardened portion and the unhardened portion of the part.
2. Discussion of the Related Art
The wear surface of certain machine parts, such as crankshafts, axles, gears, piston rods, shafts, bearing sleeves, ball bearings, tapered bearings, etc., is sometimes case hardened so that the part has a longer life and does not readily fail. Hardness is an intrinsic property of a material that is a result of the fundamental crystalline structure of the material. The CRC Handbook of Chemistry and Physics defines hardness as a property of substances determined by their ability to abrade or indent one another. Case hardening is typically provided by thermally treating the wear surface of the part, or providing some form of atomic displacement (stress), to add strength to the surface of the part. Typical hardened depths of a part are on the order of 0.5 mm-1 mm.
In order to ensure that a part is properly hardened, indenter hardness measurement systems are employed to test the part to determine the hardness and case depth of the hardened portion. International standards have been established for the precise procedures and equipment to be used in performing such hardness tests. A typical hardness measurement system that measures the hardness of a surface of a part employs an indenter tool having a tip that indents or pits the surface of the part under a known load. The tip of the indenter tool can have any one of several shapes, including spherical, diamond, pyramidal, etc. Either the displacement of the tool or the size of the formed pit in the surface of the part is then measured, depending upon the standard system used, to determine the hardness of that hardened portion.
Known hardness measurement systems that determine the depth of the case hardened portion employ processes that are typically destructive to the part. Further, the processes are costly and time consuming. Particularly, known indenter hardness measurement systems that determine the depth of the hardened portion of a part require that the part be sectioned and the sectioned surface be polished to measure the case hardness across the sectioned surface at predetermined intervals to identify the transition between the case hardened portion and the unhardened portion, and thus the depth of the case hardened portion. If the part includes multiple case hardened areas, the difficulty and expense of the measurement task increases proportionately.
In one known hardness test, referred to as a Brinell test, a load in the range of 500-3000 kg is applied to an indenter tool having a 10 mm steel or tungsten ball tip. The indentation pit made by the indenter tool is measured with a microscope calibrated in millimeters. The hardness is then calculated as:                               H          B                =                  L                                    π              ⁢                                                           ⁢              D              *                              (                                  D                  -                                                                                    D                        2                                            -                                              d                        2                                                                                            )                                      2                                              (        1        )            In equation (1), L is the value of the load, D is the diameter of the ball tip, and d is the diameter of the indentation pit.
In another known hardness test, referred to as the Rockwell or Rockwell superficial test, the hardness of the part is determined by measuring the difference in penetration depth of the tool into the part between a light (minor) first load and a heavier (major) second load with either a spherically terminated conical diamond tip or a steel ball tip indenter tool. In other words, the microscopic distance that the indenter tool moves into the part under the first and second loads is measured to determine the hardness. In the Rockwell test, the minor load is typically about 10 kg and the major load is 60-150 kg. The Rockwell test can employ an electronic Instron Machine, model 2000 to perform the hardness analysis. This is the primary method of hardness testing under consideration in this disclosure.
In another test, referred to as the Vickers test, the indenter tip is pyramidal in shape, rather than spherical, and the loads range from 1-120 kg. The hardness number is calculated as the ratio of the load applied to the surface of the indentation by:                               H          v                =                              1.8544            ⁢                                                   ⁢            P                                D            2                                              (        2        )            In equation (2), P is the load pressure applied and D is the diagonal of the indentation.